This invention relates to optical switches and optical attenuators.
In fiber optical communication systems in use today, couplers split optical signals into multiple paths or combine signals for transmission over one path. Two fibers, each considered as an optical waveguide, are pressed closely together so that energy that leaves one fiber is, for the most part, captured and used by the other (contiguous) fiber. Assuming that no energy is lost within a fiber due to Fresnel transmission through the boundary (due to absence of total internal reflection within the fiber), a small amount of optical energy can escape form the fiber in the form of an evanescent wave, which has an amplitude that decays very rapidly with increasing distance from the fiber boundary. A fiber coupler seeks to capture this evanescent wave energy emitted by a first fiber in a contiguous second fiber. Because the evanescent energy is the same fraction of the total optical energy available at a separation gap of width Dg between the two fibers, over a characteristic optical coupling length or distance Lc, substantially all energy from the first fiber can be coupled into the second fiber. Over a second (consecutive) characteristic distance Lc, the energy coupled into the second fiber will return to the first fiber by the same mechanism. The coupling length Lc varies with wavelength and with the dimensions and refractive indices of the fiber and of the ambient medium. FIG. 1 illustrates how optical energy, initially present in a first fiber 11, is progressively coupled into a second contiguous fiber 12 over a first distance Lc and is then progressively coupled back into the first fiber over a second distance Lc. In many circumstances, it is difficult to control the relative amounts of light appearing in each of the first fiber and the second fiber beyond the coupling region shown in FIG. 1.
A single mode thermo-optic switch, disclosed recently by Photonic Integration Research, uses a modified Mach-Zehnder interferometer with equal (rather than unequal) fiber lengths between two fiber couplers that define the interferometer and provides a thin film heater adjacent to the fiber in one arm. When the heater is activated, the change in fiber temperature causes a change in refractive index of the heated fiber, which changes the effective length of the heated fiber and causes interference between light beams propagating in the two interferometer arms. The apparatus behaves as a wavelength switch for light, but with rather slow reactions, requiring switching times that are estimated to be seconds or tens of seconds.
What is needed is an approach that allows the relative amounts of light appearing in each of the first and second fibers at a selected wavelength beyond the coupling region to be controlled so that, if desired, all light appears in a selected one of the first and second fibers. Preferably, this approach should allow the relative amounts of light appearing in each fiber to be changed slowly and continuously, if desired, or to be changed abruptly. Preferably, this approach should be applicable to any wavelength within a selected range. Preferably, this approach should not require a substantial increase in the volume occupied by the apparatus vis-a-vis the volume occupied by the fibers and light source. Preferably, a reaction time for switching or attenuating light with a selected wavelength should be a small fraction of a second.
These needs are met by the invention, which provides a first approach for magnetically controlling the optical coupling length Lc through use of a magnetostrictive (MS) material that changes its optical coupling length Lc, its gap width Dg and/or the refractive indices of the two fibers within the coupling length Lc, in response to a change in strength of a magnetic induction field impressed on the material. This approach is applied to provide an optical switch or optical attenuator in which light propagating in a first optical fiber is switched on, switched off or attenuated by application of a magnetic induction of appropriate strength and orientation to the fiber.
In a second approach, a Mach-Zehnder interferometer (MZI) is provided for a pair of fibers or channels, with a first fiber, but not a second fiber, including a magnetostrictive element and the two fibers being subsequently coupled using a standard fiber coupler. An MZI includes first and second fibers extending between a first fiber coupler and a second fiber coupler, spaced apart, with the two fiber lengths between the couplers being different by a selected length difference, with the coupling coefficients preferably being 50 percent at each coupler. Light propagating in, say, the first fiber (or second fiber) may be fully transmitted, partly transmitted or blocked, depending upon the length difference, the refractive indices of the fibers and the light wavelength. When a magnetic field impressed on the magnetostrictive element is changed, transmission or blockage of light at the second coupler is changed. A second magnetostrictive element, having the same MS material or, preferably, another MS material with different characteristics, is optionally positioned in an MZI arm including the second fiber, to provide additional control over the change in refractive index and/or physical length of the first and second MS elements.
Optionally, the system used in the first approach and/or in the second approach is positioned within a temperature control module to provide improved control over the MS characteristics of the system.